Nuclear power plants typically utilize water to remove the heat created by the fission of an element such as uranium within the nuclear reactor. In a pressurized water reactor (PWR), heat is removed from the reactor by water flowing in a closed pressurized loop. The heat is transferred to a second water loop through a heat exchanger. The second loop is kept at a lower pressure, allowing the water to boil and create steam. The steam is used to turn a turbine-generator and produce electricity. Afterward, the steam is condensed into water and returned to the heat exchanger.
The Voda-Vodyanoi Energetichesky Reaktor (VVER) is the Russian version of a PWR. FIG. 1a presents an elevational view of a standard fuel assembly 2 for use with a VVER. The fuel assembly 2 contains a plurality of fuel rods 3, a plurality of grids 4, a top nozzle 6, and a bottom nozzle 8. FIG. 1b presents a close-up view of Area A shown in FIG. 1a, depicting the top nozzle 6 and top grid 5. FIG. 1c presents a close-up view of Area B shown in FIG. 1a, depicting two mid-grids 7, 9 and a portion of the fuel rods 3. FIG. 1d presents a close-up view of Area C shown in FIG. 1a, depicting the bottom nozzle 8 and bottom grid 10.
Each fuel rod 3 contains uranium oxide pellets that are stacked in cladding. A spring is positioned at the top of the stack to compress the pellets. The fuel rod is closed at both ends by end plugs that are welded to the cladding. Grid springs provide lateral support for the fuel rods 3 and accommodate for growth that occurs during irradiation. Control rods are interspersed among the fuel rods to regulate the nuclear reaction. The control rods slidably move within guide thimbles that are anchored to the grids 4 and/or nozzles 6, 8 by welding. The grids 4 are positioned one on top of the other in a tandem array, usually at regularly spaced intervals. An instrumentation tube may be positioned in the center of the fuel rods and control rods.
FIG. 2 presents a cross-sectional view of section D-D′ shown in FIG. 1a. FIG. 2 illustrates the geometric array 11 or shape in which the fuel assembly 2 is contained in a VVER. As shown, the fuel rods 3 are contained within a geometric array 11 that is shaped like a hexagon with six corners 13 (a “hexagonal array”). Control rods 14 and their associated guide thimbles (indicated by the outer circle (i.e., perimeter) surrounding each of the control rods 14 in FIG. 2) are interspersed among the fuel rods 3, and an instrumentation tube 16 is located in the center. Typically, a VVER fuel assembly with a hexagonal array will include 312 fuel rods, 18 control rods and associated guide thimbles, and 1 instrumentation tube. The structural support for the fuel assembly is provided by the grids, the top nozzle, and the bottom nozzle, which are anchored to the guide thimbles. Structural support is also provided by the grid springs which offer some lateral stability to the fuel rods. In addition to hexagonal arrays, VVER fuel assemblies may have square or circular arrays. Square fuel assemblies will typically have a 14×14, 15×15, 16×16, or 17×17 array. A 16×16 array may include 237 fuel rods, 18 control rods and associated guide thimbles, and 1 instrumentation tube.
Unfortunately, standard VVER fuel assemblies may not provide adequate geometric and dimensional stability during irradiation, or sufficient resistance to fuel assembly distortion. Fuel assembly bow and twist measurements, handling incidents, and incomplete rod insertion (IRI) events indicate that standard VVER fuel assembly designs may not adequately support current fuel management schemes with four annual cycles (i.e., four year long fuel cycles, during which time a region of fuel assemblies may remain within the reactor core). Moreover, standard fuel assembly designs may not adequately support proposed fuel management schemes with 6 annual cycles (i.e., six year long fuel cycles, during which time a region of fuel assemblies may remain within the reactor core) and maximum fuel rod burn-up of 75,000 MWD/MTU. Some fuel assemblies have been designed to include structural support straps that wrap around the assembly perimeter. These structural support straps provide an increased resistance to fuel assembly distortion. However, their design has some disadvantages associated with manufacturing problems (e.g., a significant number of weld joints) and thermal-hydraulic limitations (increased fuel assembly pressure drop; decrease in the DNB performance for fuel rods at corner locations).
Thus, there exists a need for a new fuel assembly design that provides adequate structural stability or skeletal rigidity and resistance to distortion to support current and proposed fuel management schemes without degradation of thermal hydraulic performance and without manufacturing problems. The goal is to sustain the fuel supply for as long as possible while at the same time maintaining the power rating of the nuclear reactor.